Bull. Mater. Sci., Vol. 32, No. 5, October 2009, pp. 531–536. © Indian Academy of Sciences.
Effect of cerium and thermomechanical processing on microstructure and mechanical properties of Fe–10⋅5Al–0⋅8C alloy R G BALIGIDAD and SHIVKUMAR KHAPLE* Defence Metallurgical Research Laboratory, Hyderabad 500 058, India MS received 1 September 2008; revised 30 October 2008 Abstract. The effect of cerium content and thermomechanical processing on structure and properties of Fe–10⋅5 wt.%Al–0⋅8 wt%C alloy has been investigated. Alloys were prepared by a combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot-forged and hotrolled at 1373 K as well as warm-rolled at 923 K and heat-treated. Hot-rolled, warm-rolled and heat treated alloys were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction to understand the microstructure of these alloys. The ternary, Fe–10⋅5 wt.%Al–0⋅8 wt.%C alloy showed the presence of two phases; Fe–Al with bcc structure, and large volume fraction of Fe3AlC0⋅5 precipitates. Addition of cerium to Fe–10⋅5 wt.%Al–0⋅8 wt.%C alloy resulted in three phases, the additional phase being small volume fraction of fine cerium oxy-carbide precipitates. Improvement in tensile elongation from 3–6⋅4% was achieved by increasing the cerium content from 0⋅01–0⋅2 wt.% and further improvement in tensile elongation from 6⋅4–10% was achieved by warm-rolling and heat treatment. Keywords. Iron aluminides; cerium addition; air induction melting; electroslag remelting; EPMA.
1.
Introduction
Iron aluminides are being developed for elevated temperature structural applications for temperatures up to 873 K (Mendiratta et al 1987; Stoloff 1998; Schwaiger et al 2003). They also provide potential replacement for the more expensive heat-resistant alloys such as austenitic and ferritic steels containing strategic elements like nickel and chromium. However, poor ambient temperature ductility, poor toughness and poor creep properties have limited their acceptance for many structural applications. Recent developments have indicated that the room temperature ductility of Fe–16 wt.%Al alloy can be improved by the addition of chromium (McKamey and Liu 1990). The micro-alloying of cerium to Fe–16 wt.%Al alloy has resulted in significant improvement in both strength and ductility (Yangshan et al 1995, 1996; Kratochvil et al 1999). It has been shown that the addition of carbon to Fe–(8⋅5–16 wt.%) Al alloys leads to an improved strength, creep resistance, machinability and resistance to environmental embrittlement (Baligidad et al 1997, 1998a, b; Schneider et al 2005). Recently a process route has been established to produce Fe–Al–C alloys containing reactive elements like Ce and also shown that addition of 0⋅2 wt%Ce (to Fe–Al–C alloys) leads to significant improvement in ductility (Baligidad and Radha-
*Author for correspondence (
[email protected])
krishna 2002a, b). The purpose of the present paper is to report the effect of cerium concentration and thermomechanical processing on microstructure and mechanical properties of Fe–10⋅5 wt.%Al–0⋅8 wt.%C alloy at ambient temperature and at 873 K as well as creep and stress rupture properties at 873 K and 140 MPa.
2.
Experimental
Four alloys with nominal compositions listed in table 1 were prepared by a combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR) as described elsewhere (Baligidad et al 1998a; Baligidad and Radhakrishna 2002a, b) (all the compositions are in wt.% unless otherwise specified). The 80 mm diameter ESR ingots were held in a furnace at 1373 K for 1 h and hot-forged to a thickness of 25 mm and subsequently hotrolled at 1373 K to 12 mm thick plate. The ESR hotrolled thick sections of Fe–10⋅5Al–0⋅8C–0⋅2Ce alloy was further hot-rolled at 1373 K to a thickness of 4 mm and warm-rolled at 923 K to a thickness of 2 mm. The 2 mm thick warm-rolled section was annealed at 973 K and 1273 K for 1 h and furnace cooled to room temperature. The details of hot-working are given elsewhere (Baligidad et al 1994). Longitudinal section of hot-rolled 12 mm thick sheets of ESR Fe–10⋅5Al–0⋅8C–0⋅01Ce (alloy 2) and Fe– 10⋅5Al–0⋅8C–0⋅04Ce (alloy 3) alloys as well as hot and 531
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warm-rolled 2 mm thick sheets of ESR Fe–10⋅5 Al–0⋅8C– 0⋅2Ce alloy were cut using bi-metallic band-saw blade. The cut-off sections were mechanically polished to 0⋅5 μm grade diamond powder finish and etched with an etchant composed of 33% HNO3 + 33% CH3COOH + 33% H2O + 1% HF by volume for microstructural examination by optical microscopy and scanning electron microscopy (SEM). For X-ray diffraction (XRD) studies in a Philips 3710 Diffractometer, the hot-rolled ESR ingots were machined and powder samples were prepared from the turnings ground and sieved to –220 mesh (75 μm) size. The bulk hardness measurements were made on the hot-rolled, and warm-rolled and heat-treated microscopy samples using a Vickers hardness machine with 30 kg load. Longitudinal ASTM-E 8M round tensile specimens of 4⋅0 mm gauge diameter and 20 mm gauge length and flat tensile specimens of 10 mm gauge length were prepared from hot-rolled ESR ingots of alloys 2–4 as well as from warm-rolled and heat-treated ESR ingot of alloy 4. Tensile tests were carried out at room temperature and at 873 K in an Instron Universal testing machine at a strain rate of 0⋅8 × 10–4 s–1. The details of sample preparation and testing procedure are given elsewhere (Khaple et al 2007). Constant load creep and stress rupture tests were carried out on hot-rolled ESR ingots of alloys 2, 3 and 4. For these tests specimens of 5 mm gauge diameter and 25 mm gauge length were machined and polished. The details of sample preparation and creep testing procedures
are given elsewhere (Khaple et al 2006). The tensile fracture surfaces were examined in SEM. 3.
Results and discussion
Scanning electron micrographs of hot-rolled ESR ingots of alloys 2 and 3 are shown in figures 1 and 2. For the purpose of comparison, micrographs of alloys 1 and 4 of hot-rolled ESR ingots are included from our previous work (Baligidad and Radhakrishna 2002b). Scanning electron micrographs showed the presence of significant amounts of precipitates in the hot-rolled ESR ingots of alloys 1–4 (figure 1). The back scattered electron micrographs of SEM revealed the presence of two types of precipitates in alloys 2–4 (figure 2) whereas alloy 1 exhibited only one type of precipitates. The matrix and large volume fraction of precipitates, which appear relatively dark in BSE images of SEM in alloys 1–4, are confirmed to be Fe–Al(α) and Fe3AlC0⋅5 phases, respectively by X-ray diffraction (figure 3). The fine, very small volume fraction of bright precipitates observed in BSE images of SEM in alloys 2–4, were identified previously, by one of the authors, as cerium-oxycarbide precipitates by EPMA (Baligidad et al 1998b). The precipitates were distributed uniformly throughout the matrix in the hot-rolled ingots of alloys 1–4. In case of cerium addition to the hot-rolled Fe–10⋅5Al–0⋅8C alloy, the carbide particles were much finer and more globular. It indicates that addition of
Figure 1. SEM secondary electron micrographs showing large volume fraction of precipitates in hotrolled ESR ingots of (a) Fe–10⋅5Al–0⋅8C, (b) Fe–10⋅5Al–0⋅8C–0⋅01Ce, (c) Fe–10⋅5Al–0⋅8C–0⋅04Ce and (d) Fe–10⋅5Al–0⋅8C–0⋅2Ce alloys.
Effect of cerium and thermomechanical processing on Fe–10⋅5Al–0⋅8C alloy
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Figure 2. SEM back-scattered electron micrographs showing large volume fraction of dark precipitates in hot-rolled ESR ingots of (a) Fe–10⋅5Al–0⋅8C alloy and large volume fraction of dark precipitates and small volume fraction of bright precipitates in hot-rolled ingots of (b) Fe–10⋅5Al–0⋅8C–0⋅01Ce, (c) Fe– 10⋅5Al–0⋅8C–0⋅04Ce and (d) Fe–10⋅5Al–0⋅8C–0⋅2Ce alloys.
Figure 3. XRD traces using Cu-Kα radiation showing bcc and Fe3AlC0⋅5 peaks in a. Fe–10⋅5Al–0⋅8C and b. Fe–10⋅5Al–0⋅8C– 0⋅2Ce alloys. Similar peaks were observed in Fe–10⋅5Al–0⋅8C– 0⋅01Ce and Fe–10⋅5Al–0⋅8C–0⋅04Ce alloys.
cerium has influenced the size and morphology of the carbides. This can be explained based on the well-known
effect of Ce addition in modifying the carbide/graphite morphology in cast iron. All the hot-rolled (alloys 1–4) alloys possess recrystallized grains. This was expected because the hot-rolling of the alloys was preformed at high temperatures (1373 K), where dynamic recrystallization has taken place. The microstructure of warm-rolled and heat-treated ESR ingot of alloy 4 (Fe–10⋅5Al–0⋅8C– 0⋅02Ce) showed very fine and more uniformly distributed Fe3AlC0⋅5 precipitates as compared to hot-rolled ESR ingots of alloy 4 (figure 4). The hardness values, room temperature and 873 K tensile test results of hot-rolled ESR ingots of alloys 2 and 3 as well as hardness and room temperature tensile properties of warm-rolled and heat treated ESR ingots of alloy 4 are listed in table 1. The creep and stress rupture results of alloys 2, 3 and 4 are presented in table 2. Each hardness data point reported here represents an average of 5 measurements, each tensile and creep rupture data point reported here represents an average of two tests. For the purpose of comparison, hardness and room temperature and 873 K tensile data of hot-rolled ESR ingots of alloys 1 and 4 tested under similar condition are included in table 1 from the literature (Baligidad and Radhakrishna 2002a, b). It can be seen from tables 1 and 2 that the base alloy Fe–10⋅5Al–0⋅8C (alloy 1) exhibits poor room temperature ductility and creep properties. Cerium addition to the base alloy has resulted in significant improvement in room temperature ductility. The tensile elongation of the base alloy increased significantly (1⋅8–3⋅0%) by the
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Figure 4. Optical micrographs of a. hot-rolled 12 mm thick, b. hot and warm-rolled and heat treated (973 K/1h/FC) 2 mm thick and c. hot and warm-rolled and heat treated (1273 K/1h/FC) 2 mm thick ESR Fe–10⋅5Al–0⋅8C–0⋅2Ce alloy showing Fe3AlC0⋅05 precipitates.
Table 1. Effect of cerium as well as warm-rolling and heat treatment on tensile properties of Fe–10⋅5Al–0⋅8 C alloy. Tensile properties (RT) Alloy
Composition (wt.%)
1 2 3 4a 4b
Fe–10⋅5Al–0⋅8C Fe–10⋅5Al–0⋅8C–0⋅01Ce Fe–10⋅5Al–0⋅8C–0⋅04Ce Fe–10⋅5Al–0⋅8C–0⋅2Ce Fe–10⋅5Al–0⋅8C–0⋅2Ce
4c
Fe–10⋅5Al–0⋅8C–0⋅2Ce
Processing condition
Hardness (HV)
Hot-rolled* Hot-rolled* Hot-rolled* Hot-rolled* Hot + warmrolled** + Heattreated*** Hot + warmrolled** + Heattreated****
UTS (MPa) YS (MPa)
Tensile properties (873 K)
El (%)
UTS (MPa) YS (MPa)
El (%)
315 299 310 290 302
833 897 914 844 789
720 732 738 660 594
1⋅8 3⋅0 4⋅1 6⋅4 10
318 349 300 292 –
300 323 294 275 –
60 70 82 90 –
288
765
584
9
–
–
–
*80 mm dia ESR ingot hot-forged at 1373 K to 25 mm thick and hot-rolled at 1373 to 12 mm thick plate; **80 mm dia ESR ingot hot-forged at 1373 K to 25 mm thick, and hot-rolled at 1373 to 4 mm thick, warm-rolled at 923 K to 2 mm; ***heat treated at 973 K for 1 h and furnace cooled to room temperature; ****heat treated at 1273 K for 1 h and furnace cooled to room temperature
addition of 0⋅01 wt.% cerium. By comparing alloys 2, 3 and 4 (table 1), it can be seen that the room temperature tensile elongation increased with increasing cerium content (table 1 and figure 5). The highest tensile elongation of 6⋅4% was obtained from alloy 4 which contained 0⋅2 wt.%Ce. Warm-rolling and heat treatment of alloy 4 resulted in further increase in tensile elongation from 6⋅4–10% without significant loss in strength. The increase
in tensile elongation was also observed in high temperature tensile tested hot-rolled ESR (alloys 1–4) alloys (table 1). The tensile elongation of all the four hot-rolled alloys at 873 K is significantly higher (60–90%) as compared to room temperature (1⋅8–6⋅4%). The transgranular cleavage failure was observed in the tensile samples of all the four alloys tested at room temperature. At 873 K the failure mode changed to ductile dimple (figure 6).
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Table 2. Effect of cerium on creep properties of hot-rolled Fe–10⋅5Al–0⋅8C alloy. Creep properties (873 K, 140 MPa) Alloy Alloy Alloy Alloy Alloy
Composition (wt.%) 1 2 3 4
Fe–10⋅5Al–0⋅8C Fe–10⋅5Al–0⋅8C–0⋅01Ce Fe–10⋅5Al–0⋅8C–0⋅04Ce Fe–10⋅5Al–0⋅8C–0⋅2Ce
Figure 5. Effect of cerium on tensile elongation of hot-rolled 12 mm thick sheet of ESR Fe–10⋅5Al–0⋅8C alloy.
It can be seen from table 1 that alloys 1, 2 and 3 exhibit 299–315 Hv hardness, 720–738 MPa yield strength at room temperature and 294–323 MPa yield strength at 873 K, whereas alloy 4, containing 0⋅2 wt.%Ce, exhibits slightly lower (290 Hv) hardness and poor yield strength at room temperature (660 MPa) and 873 K (275 MPa). This study indicates that addition of cerium up to 0⋅04 wt.% did not improve the strength, whereas addition of 0⋅2 wt.%Ce has resulted in reduction of yield strength both at room temperature and 873 K. It has been reported (Yangshan et al 1996) earlier that the addition of small (0⋅15%) amount of cerium to carbon free Fe–16Al alloy resulted in significant improvement (8⋅5–14⋅3%) in tensile elongation, and marginal improvement in yield strength (470–505 MPa). The reasons for poor yield strength observed in the Fe– 10⋅5Al–0⋅8C–0⋅2Ce alloy in the present work have not been understood yet. It can be seen from table 2 that addition of 0⋅04–0⋅2 wt%Ce to the base alloy (Fe– 10⋅5Al–0⋅8C) did not exhibit any significant improvement in either creep life or minimum creep rate. However, alloys containing cerium exhibited significant reduction in area accompanied by necking and elongation of 94–110% as compared to the base alloy (58%). In the present work the mechanisms by which cerium ductilizes the high carbon Fe–10⋅5Al alloy has also not been understood. However, it is felt that the ductilization by Ce may be due to the formation of fine and globular carbide (Fe3AlC0⋅5) grains or due to the improved resis-
Life (h) 2⋅4 2⋅5 2⋅6 2⋅9
MCR (%/h) 11 8⋅0 9⋅2 8⋅6
El (%) 58 94 88 110
tance to environmental embrittlement offered by the modification of surface oxide by cerium (Yangshan et al 1996). Fe–Al alloys containing more than 8 wt.%Al are known to be susceptible to environmental embrittlement in the presence of atmospheric moisture (Baligidad et al 1996; Chiu et al 2000). This embrittlement has been proposed to be due to diffusion of nascent hydrogen liberated by the chemical reaction between aluminum present in the alloy and the atmospheric moisture. Cerium addition may accelerate the formation of Al oxide which passivates the specimen surface, thus preventing hydrogen diffusion into the specimen. This may also be attributed to the grain refinement during solidification by precipitation of high melting cerium-rich particles. In the present study, further improvement in the room temperature ductility (6⋅4–10%) achieved in alloy 4 (Fe–10⋅5Al– 0⋅8C–0⋅2Ce) by hot rolling to 4 mm thickness and warm rolling to 2 mm thickness and heat treatment may be attributed to formation of very fine and more homogeneously distributed Fe3AlC0⋅05 precipitate (figures 4b and c) as compared to hot-rolled 12 mm thick ESR alloy (figure 4a) due to the breaking of the precipitate by greater amount of working or may be due to the formation of fine grains by recrystallization (figure 4). The high tensile elongation of all the four (alloys 1–4) ESR hot-rolled alloys at 873 K as compared to room temperature may be attributed to the increased dislocation mobility at higher temperature and is related to the change in failure mode from cleavage to the more ductile dimple. 4.
Conclusions
(I) As-rolled ESR Fe–10⋅5Al–0⋅8C alloy exhibits a two-phase microstructure consisting of (a) Fe–Al with disordered bcc structure and (b) Fe3AlC0⋅5 precipitates. Addition of cerium to Fe–10⋅5Al–0⋅8C alloy resulted in the same phases, in addition to small volume fraction of cerium–oxycarbide precipitate. (II) Small amount (0⋅01–0⋅04 wt.%) of cerium addition to Fe–10⋅5Al–0⋅8C alloy resulted in a significant improvement (1⋅8–4⋅1%) in room temperature tensile elongation. Further improvement in tensile elongation from 4⋅1–6⋅4% was achieved by increasing the cerium content from 0⋅04–0⋅2 wt.%.
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Figure 6. SEM fractographs showing the change in tensile fracture mode from transgranular cleavage (a & c) at room temperature to ductile dimple (b & d) at 873 K in hot-rolled ESR. (a & b) Fe–10⋅5Al– 0⋅8C and (c & d) Fe–10⋅5Al–0⋅8C–0⋅2Ce alloys.
(III) Addition of cerium (0⋅01–0⋅2 wt.%) to Fe–10⋅5Al– 0⋅8C alloy did not result in any significant improvement of the room and high-temperature strength as well as creep properties. (IV) Warm-rolling and heat treatment of ESR hot-rolled alloy Fe–10⋅5Al–0⋅8C has resulted in further increase in tensile elongation from 6⋅4–10%. Acknowledgements The authors are grateful to the Defence Research and Development Organization, Ministry of Defence, New Delhi, for financial support in carrying out this research work. The authors wish to thank Dr D Banerjee, DRDO, New Delhi, Dr G Malakondaiah, Director, DMRL, for their interest and encouragement. The authors would like to thank fellow officers and staff of various groups of DMRL such as ERG (melting and casting), MEG (sample making), MWG (forging and rolling), MBG (tensile and creep testing) and SFAG (Metallography) for support. References Baligidad R G and Radhakrishna A 2002a J. Mater. Sci. 37 5021 Baligidad R G and Radhakrishna A 2002b J. Mater. Sci. Lett. 21 1231 Baligidad R G, Prakash U, Radhakrishna A, Rao P K and Ballal N B 1994 Ironmaking Steelmaking 21 324
Baligidad R G, Prakash U, Radhakrishna A, Ramakrishna Rao V, Rao P K and Ballal N B 1996 ISIJ Int. 36 1215 Baligidad R G, Prakash U and Radhakrishna A 1997 Mater. Sci. Eng. A231 206 Baligidad R G, Prakash U and Radhakrishna A 1998a Mater. Sci. Eng. A255 162 Baligidad R G, Prakash U and Radhakrishna A 1998b Intermetallics 6 765 Chiu L H, Lee P Y and Chang C L 2000 Mater. Manuf. Process 15 807 Khaple Shivkumar, Baligidad R G and Rao S 2007 Mater. Sci. & Technol. 23 930 Khaple Shivkumar et al 2006 Technical Report 410, DMRL, Hyderabad, India Kratochvil P, Karlik M, Hausild P and Cieslar M 1999 Intermetallics 7 847 Mendiratta M G, Ehlers S K, Dimiduk D M, Kerr W R, Mazdiyasni S and Lipsitt H A 1987 in High-temperature ordered intermetallic alloys II (eds) N S Stoloff et al (MRS: Pittsburgh, Pennsylvania), Vol. 81, pp 393–405 McKamey C G and Liu C T 1990 Scr. Metall. 24 2119 Schneider A, Falat L, Sauthoff G and Frommeyer G 2005 Intermetallics 13 1322 Schwaiger A et al 2003 Metal Powder Report 58 32 Stoloff N S 1998 Mater. Sci. Eng. A258 1 Yangshan S, Zhengjun Y, Zhonghua Z and Haibo H 1995 Scr. Metall. Mater. 33 811 Yangshan S, Zhonghua Z, Feng X and KunZhong W 1996 J. Mater. Sci. Lett. 15 820
